The principal hypothesis tested in this project is: Age-related changes in musculoskeletal function produce modifications in gait that affect economy of walking. Economy is operationally defined as the amount of oxygen consumed (per kilogram of body mass) to walk a given distance. To evaluate the above hypothesis, the project is divided into three phases, with each phase examining a related but more specific hypothesis. The first phase examines the hypothesis that older adults, particularly those who maintain a sedentary lifestyle, expend more energy to walk a given distance regardless of speed than do younger adults. This hypothesis will be tested by examining the economy of treadmill walking at six speeds (between 0.54 and 1.88 m/sec) in four groups of subjects: physically active old (70-80 yrs) and young (20-30 yrs) and physically inactive old and young. The second phase examines the hypothesis that, although economy of walking decreases with advancing age, older adults adopt patterns of gait that are as economical as possible given their physical and physiological limitations. To test this hypothesis, the effect of stride length variations on economy will be examined at three speeds of walking: 0.54, 1.21 and 1.66 m/sec. In addition, a related kinetic analysis of total body work and angular impulse (for overground walking) will be made to identify sources of economy differences between old and young and physically active and inactive individuals. The third phase examines the hypothesis that improvements in musculoskeletal function (dynamic muscle strength and joint range of motion in the legs) will affect changes in the pattern of gait that improve economy in the aged. This hypothesis will be tested by assessing speed-economy and stride length-economy before and after a 16-week training program designed to improve strength and range of motion at the ankle, knee, and hip joints. The combination of these analyses represent a unique approach to understanding how gait patterns are influenced by advancing age, and they should yield a much needed understanding of the musculoskeletal variables that impair economy and perhaps underlie pathomechanics of gait frequently observed in the aged. Moreover, with such a principled empirical base, it may be possible to distinguish between adaptations in gait that might be considered a normal concomitant of aging and impairments in gait that might be remediated with improved musculoskeletal function. Such information could be valuable in designing gait training and rehabilitation programs in the aged.